[Searchable Snapshots] Use _primary as the default search preference for searchable snapshot indices

[kotwanikunal]

Is your feature request related to a problem? Please describe.
Searchable snapshots enable querying of indices stored within the snapshots on repositories by only fetching data needed by the query on-demand. It utilizes a file cache for enabling downloads and tracking frequently used pieces of data called as blocks. Queries can have a better latency if the same shard is queried for repeated/similar queries since the cache efficiency will be higher.

Describe the solution you'd like

• Use primary as the default search preference for searchable snapshot indices
  • The default routing of requests for searchable snapshot indices should have a _primary preference
  • This will ensure that only the primaries answer the queries for that particular shard, leading to a high cache efficiency

Describe alternatives you've considered
Described as a part of #7436

Additional context

• Add back primary shard preference for queries #7375
• [Searchable Snapshots / Segment Replication] Custom query routing for performance improvements #7436
• [DISCUSS] Add back preference for searching _primaries or _replicas #6046